Phosphoric acid electrolyte baths for electrochemical decontamination have been used for several years. After long term usage, such baths show an increased iron content and activity in the electrolyte sleuthing. With iron concentrations in excess of 100 g Fe/l, further use of the electrolyte becomes uneconomical because the decontamination processes are very time consuming and labor intensive. Accordingly, the electrolyte must be discarded.
When the baths are used to decontaminate radioactive metallic articles, they become radioactively contaminated themselves and thus must be stored and contained with the precautions associated with radioactive materials.
Two principal techniques have been used heretofore for the treatment of radioactively contaminated aqueous phosphoric acid electrolytes.
In one approach the phosphoric acid electrolyte, containing about 30 to 40% phosphoric acid, is diluted some 50 times with water. This is necessary to prevent, during the subsequent neutralization with sodium hydroxide, a precipitation of the Na.sub.3 PO.sub.4.12 H.sub.2 O.
In a second step, the sodium hydroxide is added with intensive stirring to a pH of the solution of 7. During the neutralization, the previously soluble iron phosphate precipitates as a sediment from which the liquid phase is easily decanted.
The iron phosphate precipitate binds the greater part of the radioactivity to it so that the supernatent sodium phosphate solution has a radioactivity which lies below the limits which require consideration of the waste water as radioactive.
Nevertheless the water can be subjected to further sedimentation and flocculation processes. This approach has the advantage that for 3000 liters of an electrolyte bath, only about 1000 kg of iron phosphate must be subjected to conditioning and storage as a radioactive waste. Its drawback, however, is that the radioactive waste water from a 3000 electrolyte bath contains about 1800 kg of sodium phosphate which is equivalent to about 1500 kg of phosphate ion, a significant environment contaminant when this waste water is disposed of.
The more common approach, therefore, provides for evaporation concentration of the electrolyte solution. To protect the evaporator and prevent deposits from forming since the solution treated otherwise has caking tendencies, the acid must be neutralized to a solution of about pH 10. As a result, this method produces a mixture of the Na.sub.3 PO.sub.4.12 H.sub.2 and iron phosphate. For a 3000 l charge of the electrolyte bath, therefore, some 1000 kg of the dodeca hydrate of sodium authophosphate must be conditioned.
While this process is less prone to environmental contamination because of the release of phosphates, it nevertheless must deal with the storage and processing as radioactive wastes of large amounts of solid residue.